Progressive wave tube comprising an output cavity and a drift space



May 28, 1957 PROGRES WARNECKE El' AL SIVE WAVE TUBE COMPRISING AN OUTPUT CAVITY AND A DRIFT SPACE Filed June 27. 1950 3 Sheets-Sheet l mv. im N s" ps yavan* May 28, 1957 R. wARNl-:CKE E-r AL 2,794,143

PROGRESSIVE WAVE TUBE COMPRISING AN OUTPUT oAvTTY AND A DRIFT SPACE Filed June 27. 1950 5 Sheets-Sheet 2 lll/IA lll/A Has/v7.5

May Z8, 1957 R. wARNEcKE E'r AL 2,794,143

PROGRESSIVE wAvE TUBE coMPRrsTNG AN OUTPUT CAVITY AND A DRIFT SPACE 5 Sheets-Sheet I5 Filed June 27. 1950 Fig-4 aus N w n N mm2. ws fwn 2 R n. s

an E i @www w ws 0 rates PROGRESSIVE WAVE TUBE COMPRISING AN OUTPUT CAVITY ANB A BRFT SPACE Robert Warnecke and Oscar Dhier, Paris, France, and Werner Kleen, Madrid, Spain, assignors to Compagnie Generale de Telegraphie Saus Fil, a corporation of France Application June 27, 1950, Serial No. 170,526 Claims priority, application France July 12, 1949 6 Claims. (Cl. S15- 3.5)

Uni

Kleen, in co-pending U. S. application Serial No. 16,974,

led March 25, 1948, now Patent No. 2,580,007, for substantially improving the output.

In the aforesaid application is described a tube containing: l

l. An electron gun generating an electron beam.

2. An electron buncher in the form of a retardation line effecting a velocity and density modulation of an electron beam in such a manner that the electronic current of the beam at the output terminal is greatly modulated in density.

3. An output circuit in the form of a cavity, with which is coupled a charge to which the high-frequency power of the electron beam is yielded due to the slowing Ydown of the electrons. f

4. A collector brought to a direct-current voltage and catching the electrons after they have passed through the output circuit.

The present invention relates to a tube of this type, which contains in addition one or more of the following characteristic elements:

5. A drift space between the retardation line and the output circuit. It must be understood that the term drift space employed in the present specification means a space in which there is no interaction between the electron beam passing through this space and an electric high frequency field. This definition includes not only a space practically free from high-frequency field which always behaves as a drift space, but also a space in which there is a high frequency field and which can also be a drift space. lf, for example, a beam is propagated in the electromagnetic field of a progressive wave, but with a velocity very different from the velocity of propagation of the wave, there is no interaction between the highfrequency field of the wave and the electrons, that is to say, the velocity of the electrons is substantially un'- aifected by the high-frequency lield. This space therefore also acts as a drift space. Similarly, an interaction between an electron beam does not exist if this beam passes through a space in which a stationary high-frequency wave is propagated, and if the time of transit t of the electrons is much greater than the period of the field T. This space, in the meaning of the present specification, is also a drift space. In the present specification, this expression will generally be used to designate a space in which the high-frequency field has substantially no influence on the velocity and the path of the electrons.

The invention also prescribes an optimum length for this drift space, given by a subsequent relation.

6. Within the drift space is situated a tank cavity detuned capacitatively for the mean frequency of the signal.

atent() fice The term tank cavity means that no signal generator and no charge is coupled with this cavity.

The invention also provides:

7. Within the retardation line serving as an Iinput circuit, a coupling between the beam and the wave existing both for the fundamental component wg and for the harmonic 2m 0f the signal. The features and the advantage of this part of the present invention will be explained at a later stage.

Finally, it is proposed in accordance with theinvention:

8. To bring the input circuit and the output cavity to different potentials, also in order to increase the output of the tube.

The invention will now be described in detail, reference beingy made to the accompanying drawings in which:

Figure 1 is a longitudinal axial section through a tubeaccording to the invention;

Figure 2 is a performance curve of the tube of Figure l;

Figure 3 is a modification of the tube of Figure 1 'including an additional feature;

Figure 4 is a performance curve for known tubes of the type to which the present invention belongs; and

Figure 5 is a modified curve, similar to that of Figure 4, but illustrating the improved performance according to the invention.

The details of the first novel feature of the present invention (hereinbefore mentioned under 5), concerning the presence of a drift space between the output terminal of the retardation line (of the input circuit) and the output cavity, will rst be explained with reference to Figure 1, which shows a tube according to the present invention. ln this ligure 1, 2, 3 are the electron gun with the incandescent cathode 1, the focussing (Wehnelt) electrode 2 and the anode 3. The electron beam generated by the gun enters a retardation line 4 shown in Figure 1 in the form of a helix. It must, however, be understood that this embodiment is not limitative and that this helix could be replaced by another form of retardation line v(multivane guide, perforated-disc guide or the like). The retardation line is brought to a positive potential with respect to the cathode, which potential may be the same as that of the anode 3, the output cavity 8 andthe electron collector 9, but the potentials of these electrodes could also differ. The high-frequency generator is coupled with the input terminal of the retardation line.. In the structure shown in Figure 1, this coupling is effected by means of an output element in the form of a coaxial line comprising the external conductor 5a and the internal conductor 5b, the latter being connected to the end of the helix 4. However, any other form of coupling could be employed between the input end of the retardation line and the generator. The wave set up by the high-frequency generator is propagated along the retardation line, effecting the velocity modulation and the density modulation of the electron beam moving inside the field-of this wave at'a velocity substantially equal to that of the wave. The etect of the modulation of the beam is accompanied by an amplification of the power of the wave.

At the end of the retardation line is situated an attenuation element 6 serving to dissipate the amplified highfrequency power. There will preferably be employed as the attenuation element a graphite block of such form that there is suflicient matching between this el'ement and the line to prevent the tube from oscillating owing to an excessively strong reflection of the amplified wave at the end of the line. lf the electric iield of the wave is plotted along the retardation line a curve is obtained with the form shown in Figure 2. The high-frequency field tirst increases exponentially from the input end to the point A, at which the attenuation element commences, whereafter it commences to decrease more or less rapidly, the field reaches at the point B the level which it 'had on entering 3 the line, and between B and C there is a space which is 'substantially free from high-frequency eld.

The present invention is characterised by the presence of a drift space, preferably of a length l such that where 7l is the operating wave length in the iree space, vo the velocity of the electrons, and c the speed of light.

For example, with:

Direct-current voltage Vu= 10 kv. v=6.109 Ac1rl./s.

c=3.10'10 cm./s.

then we have l 6 cm.

The optimum value of this length corresponding to the best output of the tube has been more accurately found by calculation and experience to be:

:i to 1.5 (2) where p. is the real part of the propagation constantrof the amplified wave within the retardationiline, ,u .being fmeasured in emr-1. The value of it is easy to deduce from the dimensions of the retardation line, yfrom lthe total current of the beam and from the velocity of the lelectrons. For example, with a tube comprisinga retardation line in the form of a helix and having the following characteristics:

Diameter ofthe helix mmcalculation and experience show a value of li=0.l cmi-1, and consequently the optimum length of the drift space lies 4between 10 and 15 cm.

In `Figure l, this drift space 7 exists Within the attenuation element 6, and if thelatter is not of suicient length, there will be added to the length of this element a space 7a free from high-frequency eld, so that the relation (2) is fulfilled.

In fact, according to the curve of Figure 2 the highfrequency field does not decrease suddenly at the point A, so that 'the length of the drift space is not defined with great precision. This is why, in the relation (2) a very precise value has not been indicated, but l has :been located lbetween two limit values. In practice, the relation .(2) may be applied to the length of the spaceb'etween B and C, B `being determined by the point at which the high-frequency eld has decreased by 3 decibels .in relation to its maximum value, and Cbeing'the point at which the gap of the'output cavity 8 commences.

However, 'according tothe definition of the drift space given at the commencement of the present specification, it is not necessary that the drift space be absolutely free from high-frequency field. It is suicient to give the attenuated progressive-wave propagated in this space a propagation speed such that there is no longer any interaction Vbetween the wave and the beam, that is to say, to modify substantially the speed of propagation of the wave in relation to its value in that part of the retardation line in which there is no attenuation apart from a very weak unavoidable attenuation. This is done automatically due to the presence of the attenuation element, which represents a charge in the line such that the wave velocity isV substantially reduced. It this effect is still not .sucient to iprevent interaction between the beam and the'wave, the retardation of the wave may be further increased by givingthe helix a 'variable pitch, in which case the pitch must b'e materially reduced within the attenuation lelement. In "brief, the present invention provides between the output end of the input retardation line andthe output cavitya space in which there is no interaction Ybetween the wave and the beam. Preferably, this space, which i's called a drift space, must have an optimum length given by the relation (2) above. The various means have been indicated by which interaction between the beam and the wave is avoided by providing a space actually free from high-frequency field or by modifying the velocity of the wave in such a manner that it is substantially different (by at least 10%) from the velocity of the electrons in this space.

Figure l also shows the other elements of the tube, notably the output cavity 8 and the collector 9, the whole being enclosed in an envelope 12.

As has already been mentioned, the presence of the drift space, and more especially of a space having a length based on the relation (2), has the advantage that it increases the efficiency of the tube. This fact is based on the following phenomena:

The retardation line used as an input circuit for the tube, due to the interaction between the wave excited therein and the electron beam simultaneously produces a density modulation and a velocity modulation of the `electronic current. For an optimum value of the tinput power (this value being determined by the dimensions of the line, by the current and by the velocity of the beam) 4it is possible to obtain at the output end of the line an .alternating current Whose amplitude is substantially equal to that of the direct current.

By injecting this alternating current into the output cavity, there is consequently obtained an electronic eii ciency of the order of 50%, which value is reduced in practice to 40% or even to lower values owing to the irnperfections of the output cavity (losses ofthe said cavity, and iinite transit angle in the gap of the cavity). By injecting this alternating current into the output cavity immediately beyond the output end of the retardation line, only the density modulation set up bythe interaction between the beam and the wave is exploited. However, this interaction within the retardation line results in a velocity modulation. The drift space beyond the helix then makes it possible to convert this velocity modulation into a density modulation, which gives a supplementary alternating current superimposed on the alternating current delivered at the output end of the retardation line. The total current is of maximum value at the distance l beyond the output end of the line, which distance is determined by the relation (2). The amplitude ofthe total alternating current at this point reaches a value which is from 1.2 to 1.4 times the direct current, which fact leads, in the case of an ideal cavity, to an electronic etciency of from 60% to 70%. This value is reduced by the imperfections of the cavity, which have been referred to in the foregoing, but nevertheless, as a result of the presence of a drift space beyond the output end of the retardation line the eiciency is substantially higher than in a tube having no drift space. The increase in the etiiciency is Amaximum if the length of the drift space is determined by the relation (2).

The bunching of the electrons, that is to say, the ratio between the alternating current and the direct current, may be further improved by introducing a tank cavity into the drift space. The principle on which this idea of the invention is based is illustrated in Figure 3, which shows the essential parts of the tube aimed at by the invention. In this gure, the numerals have the same significance as those of Figure l.

The drift space is here composed of two parts 13 and 7b, of which the part 13 may itself lbe constituted as before of two parts 7 and 7a. The parts 7 and 7a correspond entirely to the spaces 7 and 7a of Figure 1. According to the invention, the third part 7b of the drift space is separated from the parts 7 and 7a by the vgap 10 of a tank cavity 11, that is to say, of a cavity with which neither a generator nor a charge is coupled.

The said cavity is detuned capacitatively for theinput signal w, the detuning being approximately such that the phase angle is comprised between 60 and 80. Due to the presence of this detuned tank cavity within the drift space, an increase in the bunching of the electrons, that is to say, an increase in the alternating current in the gap of the output cavity, is obtained. This effect is produced in the following manner: Owing tothe passage of the electronic current through this cavity, the latter is energised, and a high-frequency eld is set up in the cavity. This high-frequency field exerts on the velocities of the electrons such a reaction that, with a highly capacitative detuning of the tank cavity, the number of electrons participating in the bunching is increased, that is to say, the high-frequency electronic current is increased. Whereas according to the preceding statements the maximum ratio between the alternating electronic current and the direct electronic current without the tank cavity is approximately from 1.2 to 1.1, this ratio can be increased to 1.5 to 1.6 by the provision of the tank cavity. It is obvious that this increase in the high-frequency electronic current brings about an increase in 4the efficiency of the tube. In addition, this method of increasing the electronic current by introducing a tank cavity has practically no influence on the pass band of the tube. On the other hand, the introduction of a tuned cavity into the path of an electron beam always has the effect of reducing the pass band of the amplifier, representing a serious disadvantage in the case of a transmission with a very wide pass band (as in the art of television for example). This disadvantage does not arise in the caseof the invention owing -to the detuning of the tank cavity.

As a third means of increasing the ratio between the alternating electronic current and the direct electronick current, the present invention proposes a particular dimensioning of the retardation line serving as an. input circuit, so that the coupling between the guided wave and the beam is of the same order of magnitude for the frequency of the signal and for its second harmonic. TheV object of this dimensioning is as follows: In order to obtain the desired modulation of the electron beam in density and in velocity within the retardation line, there musty be a coupling between the retardation line and the beam. This coupling may be dened by indicating the longitudinal electric field Ez on the axis of the line in the case where a unit of energy is propagated in the form of electromagnetic energy along the line. By plotting this value of Ez, that is, the magnitude determining the coupling between the beam and the wave as a function of the frequency w, it is found that Ez depends considerably upon the frequency. In general, there is obtained for Ez=f(w) a curve such as is plotted in Figure 4, which shows that for the frequency of the signal ws, Ez is fairly i high, the coupling is tight, while for the harmonics 2ws, 3ws, Ez is very low with respect to its Value for W. On the contrary it is proposed lin accordance with the present invention, that the line be so dimensioned as to give the coupling as a function of the. frequency a rate of change such as is indicated by the curve I of Figure 5. This rate of change is characterised by the fact that for. the frequencies ws and Zws, the coupling is of the same order of magnitude, while in general, as will be seen from Figure 4, this coupling is substantially lower and practically zero for the harmonics 2ws, 3ws, of the signal with respect to the coupling for the frequency wg. In addition, it has been found advantageous to give the retardation line, in conjunction with this coupling which is stronger than usual for the harmonic Zws, a certain dispersion of propagation velocity of the wave. This is expressed by the curve 2 in Figure 5, in which is traced the phase velocity vp of the waves propagated in the retardation line as a function of the frequency w. In accordance with the invention, the phase velocity must be higher for the signal frequency ws than for the harmonic Zws, the relative velocity difference for ws and Zws being preferably of the order of 5% to 10%.

The idea and advantage of dimensioning the retardation line in accordance with the curves of Figure 5 are based on the following facts: For large signals, the process of density and velocity modulation of the electronic beam by means of the wave guided by the retardation line'is a non-linear phenomenon. This means that the velocities of the electrons as well as the high-frequency electronic current contain terms Zws, etc. If the coupling as a function of the frequency has the rate of change shown in Figure 4, the high-frequency eld of the wave contains practically only the fundamental component. For 2ws, 3wE the coupling is so weak that even in the presence of harmonic high-frequency currents, high-frequency fields of these frequencies are not excited by these electronic currents. If, on the other hand, the coupling between the wave and the beam if of the same order of magnitude for wis and 2wB (see Figure 5), there exist highfrequency fields of the wave of the frequency ws and Zws. The density and velocity modulation of the electron beam then takes place under the influence of a high-frequency field of the wave also containing the frequencies ws and Zws, which effect does not exist ifEz=f(w) has the rate of change shown in the curve of Figure 4. This novel effect, i. e. the reaction of the high-frequency field of the wave with w=ws and w=2wB has the result of improving the modulation of the beam in such a direction that the density modulation of the current entering the output cavity is increased, that is to say, that by nonsinusoidal modulation the ratio between the high-frequency current and the direct current passing through the output cavity increases. The increase in the efficiency of the tube resulting therefrom is obvious.

In practice, this coupling for the harmonic 2, preferably combined with a certain dispersion of propagation speed in the line, must be obtained by a suitable dimensioning of the retardation line. It is not possible to indicate general relations for achieving this object, having regard to the numerous diderent forms of retardation line, and the multiplicity of different operating conditions. The dimensioning of the line required to obtain the desiredV effects must be determined by experience by varying, for example, the diameter of the line, the pitch and the diameter of the wire if the line has the form of a helix, and the distance between the line and the walls of lthe tube. In order to dene this part of the present invention, it is sufficient to state that the coupling between the guided wave must be of the same order for the fundamental component w8 as for the harmonic Zw,g of the signal, and that preferably the wave propagation velocities for ws and ZwE are slightly different, that for Zws being from 5% to 10% lower than that for ws.

In the tubes shown in Figures 1 and 3, only the highfrequency power yielded to the output cavity is exploited, while the high-frequency power generated by the reaction of the beam on the wave, which is propagated in the retardation line at the end thereof, is absorbed by an attenuation element. The invention is not limited to this method of extracting the high-frequency power. The attenuation element may be replaced by an output circuit of any form and the high-frequency power of the wave generated along the retardation line may be injected into a useful charge. Also, both the power generated along the retardation line and the power yielded to the output cavity may be utilised by yielding them to the same charge, which effect is obtained by means of suitable phase shifters by the methods indicated in the patent application referred to at the beginning of this specification.

In addition, it may be advantageous to bring the different elements of the tubes shown in Figures l and 3 to different potentials. This may even be Very important and advantageous for the purpose of increasing the efficiency. In order to obtain a very wide pass band, for example, the output cavity must Ibe highly attenuated. The slowing down of the electrons is then only incomplete, that is to say, if the output cavity is at the potential of the retardation line the electrons leave the output cavity with fairly great Vmean kinetic energy, which energy Vis yielded to the collector Yand 'converted into heat. In this case, the total eiciency of the tube -is increased the collector or 'the output cavity and the collector, are brought to lower potentials so selected 'that -allfthefelectrons can still be kcaught by the collector despite the reduction in their velocity by the high-"frequency'eld ofthe output cavity.

The general principle of the present 4invention resides in increasing the degree fof modulation in tubes within which, in accordance `with the aforesaid U. S. patent application, a retardation line is usedasafmodulator for an electron beam and an output cavity lis employed, yto which the .highfrequeucy power .of the beam is yielded. The Yinvention is not llimited to the forms of tubes illustrated by way of example in Figures .l and 3, but is generally applicable to the `.assemblies described in the aforesaid patent application, Various .forms of `retardation line, different methods -of Ycoupling the generator with the retardation vline and any form rofoutput cavity may be employed. The invention is also equally applicable to tubes cooled by radiation, by air or by water.

We claim:

1. An electron discharge tube for ultra-short wave operation having a first, amplifying section comprising an electron gun positioned to emit an electron beam along a predetermined trajectory, a 'delay line extending parallel to said trajectory in coupled relationship with said beam and having input and output ends, the delaying characteristics of said delay line'being such as to cause the phase propagation velocity of a wave travelling in said line to be ysubstantially equal to the velocity of electrons in said beam, and coupling means adjacent said gun for supplying radio-frequency energy to said line at said input end thereof for interaction with the electrons of said beam; a second, dissipating section for absorbing radio-frequency energy carried by said line comprising absorbing means positioned in the vicinity of said output end remote from said gun; a third, transfer section for extracting radiofrequency energy carried by said beam comprising a cavity resonator 'having an input aperture for said beam, coupling means to said resonator for extracting amplied radio-frequency energ/ and transferring said amplified energy 'to an outside circuit, and collecting means for the electrons of said beam; and a fourth, electron bunching section interposed between said second and third sections comprising -means defining a drift space in said trajectory intermediate said absorbing means and said aperture, said drift space being free of interaction of said electrons with a high frequency field and of a length to convert the velocity modulation imparted to said electrons by interaction with said wave in said amplifying section into an emphasized density modulation.

2. A tube as in claim 1 wherein said drift space is of such length and said delay line is of such propagation constant at the operating frequency of the tube that the product of said length by the real part of said constant Y8 is comprised :between 1 vand 1.5, said length and constant being stated in homogeneous units.

3. `A tube asincl'aim l `-further comprising an unloaded cavity .resonator having a gap lpositioned in the electron trajectory through said drift space d'e'ning means.

4. A'tub'e as -in claim 1 wherein said absorbing means comprise attenuating means located inside the tube Vin the vicinity of vtheoutpu't fend of said line.

5. VA Stube Yas in claim 1 in combination with means for applying'diiferentpotentials .respectively to said delay line and to said cavity resonator.

6. A11 electron discharge 'tube for ultra-short wave operationihavingcarst, amplifying section comprising an electron gun positioned to emit an electron beam along a predetermined"trajectory, a delay line Vextending parallel to said trajectory in coupled relationship with said beam and having input and outputY ends, the delaying characteristics of said delay line being such as to causethe phase propagation velocity of a wave travelling in said line to be substantially equal to the velocity of electrons in said beam, and coupling means adjacent said gun for supplying radio-'frequency energy to said line at said input end thereof for interaction with the electrons of said beam; a second, dissipating section for absorbing radio-frequency energy carried by said lline comprising absorbing means positioned in `the vicinity of said output end remote from'said gun; a `third, transfer section for extracting radio-frequency en'ergycarried by said beam comprising a cavity resonator having an input aperture for said beam, coupling means to said resonator for extracting amplified radio-frequency energy and transferring said amplified energy to an outside circuit, and collecting means for the electrons of said beam; and a fourth, kelectron bunchiug section interposed between said second and `third sections comprising means defining a drift space in said trajectory intermediate said absorbing meanszand said aperture, said drift space being free of high frequency y'held other than :that carried by said beam and of an axial dimension `greater than the maximum transverse dimension thereof, thereby to emphasize ythe buncliing of electrons prior to Vtheir passage through said aperture.

References Cited in the le of this patent UNITED STATES PATENTS 2,541,843 Tiley Feb. 13, 1951 2,575,383 Field Nov. 20, 1951 2,580,007 Dohler Dec. 25, 1951 2,636,948 Pierce Apr. 28, 1953 FOREIGN PATENTS 934,220 France Jan. 7, 1948 OTHER REFERENCES Abstract of Comptes rendus des seances de LAcademie des Sciences, vol. 229, pp. 648-649, seance du 3 Octobre 1949. 

